Whatever you post on social media, it's now public. Yes there are ways to make posts private and such, but glitches do happen. Not only that, but hackers may be able to turn privacy features off. Also, sometimes the social media company changes policy on some aspects, which may turn some private features into public ones. That means you should be careful not to post anything too personal and also don't post anything such as addresses, bank account numbers, passwords, etc. This is probably common sense to you, but it's a good thing to keep in mind regardless.
Also, if there are people who verbally threaten or harass you, then you should either report them or block them. Preferably both, but blocking is probably more effective. If someone friend requests you, then it's ideal to only accept their request if you know who they are. Though you can relax this if you aren't worried about who sees your profile page.
In addition, you should be careful as to what you read on social media. Not everything is true. This is because practically anyone can post a news story even if it's completely made up. Lately there have been some efforts to do fact checking, but there's still a lot of fictional stuff out there. Be sure to avoid clicking on any links you may not recognized as they could be phishing links, or may lead to harmful malware. It's probably just best to get an antivirus of some kind.